Since the dawn of Equestria, the Agency has protected the realm from every manner of hideous magical creature. Largely by putting them various places where ponies probably won't go.
Forcibly recruited via a burlap sack, Fluttershy is assigned to inventory a site that has become a center of magical gastropod evolution--only to have her plane shot down by a mysterious unicorn.
Injured and trapped in the Forest of Snails, she finds that there was far more in those swampy, moist lands than she was led to believe.